Japanese Patent 3217680, which is counterpart of U.S. Pat. No. 5,657,736, discloses a fuel metering control system in order to decrease a deviation of the air-fuel ratio between the cylinders to enhance an accuracy of air-fuel ratio control. In the fuel control system, a model showing an activity in the exhaust system is established. An air-fuel ratio sensor is disposed at a confluent portion of exhaust manifolds to detect an air-fuel ratio of an exhaust gas. The value of the air-fuel ratio detected by the air-fuel ratio sensor is input into the model. An observer estimates an air-fuel ratio of an air-fuel mixture in each cylinder, and corrects a quantity of fuel injected into each cylinder according a deviation between the estimated air-fuel ratio and a target air-fuel ratio, so that the air-fuel ratio in each cylinder is closed to the target air-fuel ratio. The period from the time in which the exhaust gas comes around the air-fuel sensor to the time in which the air-fuel ratio is detected varies according to an operation condition of engine. The period is referred to as a response delay in exhaust system. The response delay in exhaust system varies according to the operation condition of engine. The relationship between the response delay in exhaust system and the operation condition of the engine is stored in a map at the time of designing and producing the engine, on which a sample timing of the air-fuel ratio sensor corresponding to a detecting timing of the air-fuel ratio sensor is varied. The response delay in exhaust system is varied according to not only the operation condition of engine but also ageing of the air-fuel ratio sensor.
JP-10-73049A, which is a counterpart of U.S. Pat. No. 5,806,506, disclose an engine control system in which a deteriorate parameter indicative of ageing is calculated by measuring the response delay after fuel injection is cut, and the sample timing of the air-fuel ratio sensor is varied based on the deteriorate parameter and the operation condition of engine.
However, in the conventional systems, it is difficult to accurately correct the sample timing of the air-fuel ratio sensor, because the response delay varies depend on each engine. The length of the exhaust manifold, which is from an exhaust port to the air-fuel sensor, varies every cylinder, and the flow stream of exhaust gas in each cylinder intricately varies according to the engine speed and the quantity of air filled in the cylinder. Thus, it is difficult to establish a precise map on which the relationship between the response delay in exhaust system and the operation condition of engine is mapped.